There are many industrial processes where it would be useful to measure sugar content of a solution, typically an aqueous solution.
One notable field where this is useful is during the manufacture of biofuels. In this case, various processes are used to obtain biofuels from plant matter (bio-mass). Plant matter contains cellulose and hemicellulose as the major polysaccharides. Upon hydrolysis, these polysaccharides are converted to glucose and xylose respectively, and both of these sugars can be converted to ethanol. In addition, certain types of plant matter (sugar cane, sugar beet etc.) contain more complex sugars such as sucrose and fructose which can also be converted to ethanol. The produced ethanol can then be mixed with petrol to form the bio-fuel gasohol. During chemical processes such as these, it is advantageous to be able to monitor the sugar content of the processed solutions.
In many applications, although not all, there is a need to measure the amount of sugar continuously in a flowing sample of the a solution, for example as it flows through a pipe. This is typically the case for processes having high throughput. The manufacture of biofuels is an example of where measurement of a flowing sample is needed.
In general in such applications, the ion concentration of the solution can fluctuate unpredictably. The solution may also contain solid material and/or gas bubbles. This can affect various types of measurement, so it is desirable to develop techniques in which the impact of dissolved ions and/or solid material and/or gas bubbles is reduced or removed.
Various techniques for measuring the sugar content of an aqueous solution have been attempted, for example as follows.
Density measurements of an aqueous solution made using, for example, a vibrating tube density meter can determine the sugar concentration if the solution contains predominantly sugar and has a low ion concentration. However, as the ion concentration increases, and/or additionally contains solid material or gas bubbles, a density measurement is liable to give the wrong sugar concentration.
Optical measurements that detect the characteristic absorption bands of the sugar molecules can also be used to determine the sugar concentration. But such techniques require optical access to the solution and the results may be adversely affected by any solid material that reduced optical transmission. The presence of particulate and bubbles can cause dispersion of light, attenuating signal and introducing noise. In practice, this will limit the size of the sample cell and the cross section that may be analyzed. Optical measurement is less reliable since it is prone to coating of windows. In addition, different sugar molecules will produce different optical absorption spectra and so the analysis may be difficult if a mixture of sugars is present in the solution.
Electrical measurements can be made through the wall of a pipe if it is non-metallic. Such electrical measurements will be relatively unaffected by the presence of any gas bubbles and solid material. The measurements can be very fast, allowing for data processing to average signals and remove transient effects caused by the passage of bubble and solid material through the sensing region. However, the electrical permittivity of the solution will generally change as both the sugar content and the water salinity vary. So it is not evident that electrical measurements permit the sugar concentration to be unambiguously determined.
More complex measurements may be taken using techniques such as Fourier Transform Infrared (FT-IR), ultraviolet detection (UV), liquid chromatography (LC), ion chromatography (IC) and LC-mass spectrometry (LC-MS). However, such complex techniques rely on expensive, complex instrumentation. These systems are generally not suitable for inline process monitoring due to their size and requirement for sampling front-ends which require filtration and are prone to clogging. Chromatography and other analytical instrumentation generally have a reasonable response time while the instrument samples, separates and analyze the sample. Chromatography typically takes minutes to separate complex chemical matrices, and FT-IR requires post-processing in software as well as searching and comparison with spectral libraries.
There is a very extensive literature describing the measurement of the glucose concentration in various body fluids, particularly blood, for medical purposes. This is of great importance in the control of diabetes, or monitoring the administration of sugar in water to patients or animals by infusion, for example. However, such techniques are generally applied to small samples that have a relatively low concentration of glucose, for example a few g/L. In an industrial context, it is desirable to use a technique that permits measurement of much higher sugar concentrations.